Several groups of motoneurons and sensory neurons have been physiologically identified in the neural control of feeding in the opisthobranch mollusc Navanax inermis. The anatomy of individual buccal neurons is being pursued by intracellular dye injections, using Lucifer Yellow, cobalt acetate, and HRP. Serial reconstruction of the buccal ganglia from thick sections can be utilized to identify these same neurons by their position, disposition of neurites, and somatic size and shape. The ultrastructure of portions of identified neurons can be determined from this section series through remounted thick sections, or by using an intracellular marker such as HRP. Serial reconstruction can be a powerful tool in exploring the synaptology of many identified neurons simultaneously - even in a ganglion containing 240 cells. The physiology of these identified motoneurons reveals characteristic patterns of coupling and uncoupling in response to sensory inputs which have been postulated to involve shunting by inhibitory synapses which are near the site of coupling and formed by sensory neurons. Structural investigations of the motoneurons have localized the probable sites of this interaction at the distal edges of the buccal ganglia and the sensory synapses can be revealed with HRP labeling. Freeze fracture studies and conventional transmission EM studies will be combined to explore the morphological basis of the electrical coupling between cells in the buccal ganglion. Gap junctions between isolated pairs of early blastomeres from the teleost, Fundulus, will be utilized to estimate the channel conductivity per structural unit in the junction. Junctional conductance (gj) is measured physiologically; then the same pair of cells is serially sectioned to reconstruct the size of the gap junction. The density of channels can be determined from freeze fracture studies. The conductance of a single channel is the ratio of the gj to the number of channels in the reconstructed junction, assuming that all channels have uniform properties.